A servo controller of an optical disk device typically controls an actuator to trace tracks of a disk and maintains a predetermined distance between the actuator and the disk. The servo controller is configured to detect the frequency characteristic of the actuator to feed it back (feedback loop).
However, the frequency characteristic of the actuator may vary due to, for example, a defect of the actuator or deterioration of the optical disk. This may decrease frequency margin or gain margin of the entire loop and, thereby, may cause oscillation. The oscillation may cause the actuator to be displaced from the track of the disk or an undesirable variation of the distance between the actuator and the optical disk. The likelihood that the actuator will be displaced by the oscillation and/or an undesirable variation of the distance between the actuator and the optical disk may be present may be reduced by, for example, pre-checking and corresponding actions.